1. Field of the Invention
The invention relates to a linear actuator.
2. The Prior Art
The explanation of the invention departs in a linear actuator of the type comprising a linear sliding tubular activation element. A linear actuator comprises an electric motor, which over a transmission drives a spindle. On the spindle is mounted a spindle nut, to which the tubular activation element is secured with its one end. The other end of the tube section is secured with a front fixture to the movable element in the construction, in which the actuator is built in, while the actuator with a rear fixture is secured in the stationary part of the construction or vice versa. During the fastening of the spindle nut, it will either move outwards or inwards on the spindle depending on the direction of rotation, and the tube section thus moves axially outwards or retracts in a manner corresponding to the piston rod on a telescopic cylinder.
Determination of the position of the tube section is typically done by counting the number of rotations of the spindle. The number of rotations multiplied by the thread pitch of the spindle determines the position of the tube section. An absolute determination of the position can be carried out by means of a rotary potentiometer, which over a gear is driven by a motor. When the gear ratio is known the number of rotations done by the spindle can be calculated. A more prevalent way of determining the position is by using a magnetic encoder. A magnetic encoder comprises a magnetic frame yoke with a number of poles, which activate a Reed-switch or a Hall-sensor every time one of the poles passes by the switch, alternatively the Hall-sensor, at which an electric signal is discharged.
For a more detailed description of the Reed switch is referred to U.S. Pat. No. 2,264,746 to Ellwood. In short, a Reed-switch is an electro mechanical component, in which a contact set, consisting of two springy metal terminals, forms an electric connection under the influence of a magnetic field. As it is a component with movable parts it will limit the physical dimensions, choice of materials, life span and operation velocity.
In U.S. Pat. No. 7,003,828 B2 is shown an actuator system with an arrangement for position detection, consisting of a pulse provider in the form of a Reed-switch, influenced by a magnetic frame yoke. For the system the limitation outlined in column 3 line 61 to column 4 line 3 applies. The imperfection is that the inertia in the mechanical system leads to unregistered counting steps. The imperfection arises in that the counting system depends on an activation of the control buttons. For one thing to indicate the direction of movement and with that, if the counter should count up or down, but also in order for the counter basically to register and count the activations of the Reed-switch. As the imperfection over time will accumulate, a reset function of the counting cycle is introduced in a well-defined position, namely when the piston of the actuator is fully retracted.
Unlike a Reed-switch, a Hall-sensor is a non-mechanical component, which varies its output voltage depending on the strength of a printed magnetic field. It is distinguished between Hall-transducers, which respond with a voltage analogously proportional to the strength of a printed magnetic field and Hall switches, which discharges a digital signal corresponding to whether a magnetic field of a given strength is printed or not. This will typically appear by a voltage level on the exit of the component, which jumps between a low level close to 0 volt and a high level, close to the supply voltage of the component, without being in other states than these two extremes. Even though the signal level of the exit here is described as a source system it is, however, not excluded that output can be of the open collector type, which is a drain system. It is intended not to be able to supply an output voltage, but serve as an electronic contact with connection to the negative pole of the supply voltage. By fitting out with extern components the output signal can be designed in preparation for adaptation for interface with micro processor or another counting mechanism. Internally in the Hall switch mechanisms are implemented, which counteract bounce by defining a suitable hysteresis for the transition states. This counteracts the need for calibration, as the number of erroneous counting pulses is minimized. Simultaneously a Hall sensor is a fully semiconductor based component with the advantages in consequence in the form of size, price and life span. For further information concerning Hall sensors reference is made to manufacturers of these for instance the company, Micromagnetics.
It is therefore noted that each time a magnetic pole is lead past a Reed switch or a Hall sensor, this will release a signal. When using a magnetic encoder, with a magnetic ring with four poles, the position is thus determined for each quarter of a rotation of the spindle. Two Hall sensors can be used, at which the position is determined eight times per rotation.
Alternatively to a magnetic encoder an optical encoder may be used, which works in the same manner, namely with a rotating disk with penetrations and a light source. When a penetration passes by the light source, a light beam temporary passes through to a photoelectric cell, which with that releases a signal. On the basis of the signals, the position is calculated by means of a micro processor in the control unit.
From U.S. Pat. No. 5,224,429 an actuator system is known, where optical sensors are used for determining the position from the rotations of the motor (FIG. 6). However, it is not described here how the control unit determines the direction of rotation of the spindle. This will result in that a potential manual adjustment can influence a wrong position indication. The price level for optical sensors compared to magnetic alternatives, however, makes this solution less attractive as the marked for actuators is submitted to a heavy price competition.
An absolute determination of the position is preferred, but when Hall sensors and Reed switches are uses it is partly due to costs and partly due to reliability reasons. Potentiometers are relatively expensive and are from a purely mechanical point of view worn-out in time. This wear appears as fully or partially interruption of connection between the guide shoe of the potentiometer and the resistive distance on the potentiometer. The wear can over time cause a change in the resistive linearity of the potentiometer over the full dynamic range, which will influence the accuracy of the position determination. Furthermore, the regulation area at the ends cannot be fully used. Another and more unnoticed moment is that it takes about a year to test the reliability of a potentiometer in an actuator, which means that it is not possible just to replace a potentiometer with another kind. If the supplier changes the construction of the potentiometer, the testing must be redone.
The object of the invention is to provide a solution to the outlined problem, namely to achieve an absolute positioning without the disadvantages of rotary potentiometers.